The present invention relates to a method for producing press-shaped thermoplastic resin articles. More specifically, the present invention relates to a method for producing press-shaped articles having excellent surface smoothness and mechanical strength.
Heretofore, various methods for producing press-shaped plastics have been known. One method, the so-called SMC process, comprises impregnating a glass fiber mat with a monomer of a thermosetting resin and hardening the monomer by a polymerization reaction after subjecting the impregnated glass fiber mat to a press-shaping operation whereby a shaped article is formed. Another known shaping method, believed to be superior to the SMC process in process productivity, comprises heating a sheet or a film comprising a thermoplastic resin, to a temperature higher than the melting point of the thermoplastic resin, and feeding the treated sheet or film to a pair of mold presses, and rapidly compressing the sheet or film. However, when a crystalline thermoplastic resin, such as poly-.epsilon.-caprolactam, polyhexamethylene adipamide, polyethylene terephthalate, polybutylene-terephthalate, polypropylene or polyethylene, is applied to the process as described above, the crystalline thermoplastic resin is preferably combined with a glass fiber mat. This is because a preheated thermoplastic resin, without utilization of a glass fiber mat, begins to flow and cannot be manipulated. Nevertheless, a molded article made from a thermoplastic resin combined with a glass fiber mat suffers a serious disadvantage in that the surface smoothness of the article is poor due to the "rising up" of glass strands to the article's surface.
To improve the surface smoothness, several methods have been proposed. Examples include the use of fillers in conjunction with a glass fiber mat or laminating a thermoplastic resinous sheet without a glass fiber mat. However, these methods also have disadvantages. For instance, they are not economically desirable because the process of preparing the composite sheet becomes complex, and neither of these methods sufficiently improves the surface smoothness.
On the other hand, a preheated thermoplastic resinous sheet, without use of a glass fiber mat, can be manipulated when the preheating temperature is lower than the melting point of the resin and the surface smoothness of the molded article is improved. However, this process suffers disadvantages in that the dimensional stability of the molded article is poor and the mechanical equipment for molding is expensive because extremely high pressure is necessary. These methods are disclosed in U.S. Pat. No. 3,621,092, and Japanese Patent Early Publication (Kokai) No. 52-40588.
A method for providing an oriented sheet or film by compressing a crosslinked crystalline thermoplastic resin under a pressure between a pair of hot press mold and cooling the same to room temperature while maintaining the pressure has been proposed in Japanese Patent Publication No. 52-6314. This method may provide a transparent and tough shaped article, but the process productivity is not good and also the energy loss is not negligibly small.
Thus, an object of the present invention is to provide a method for producing press-shaped articles having excellent surface smoothness, mechanical strength and dimensional stability which are free from the disadvantages described above. Another object is to provide a method for producing such articles, which method has excellent productivity.